They went from "n, with gcd(3, n) = 1" to "3n", with the limit on "n, with gcd(3, n) = 1" being 2,500. Then the limit on "3n" is 2,500: 3n < 2,500 so n < 2,500/3 = 833.33... So it SHOULD have been "3n, with n < 833". You can see this working with the other one: 5n < 2,500, so n < 2,500/5 = 500.

Maybe this example started with "prime relative to 2" (so 2n < 2,500, n < 2,500 = 1250, that they switched to 1249 for some reason). I think they just made a mistake in the book. Plus, this is a really weird way to do Venn Diagrams.